Lego senior project manager Dale Chassy, who worked on the project with his team in the United States, said: "We were working on it for about six months, which is a quick turnaround for a model of that size. But we [thought we] can't build it in the US, so we need to partner up with Ryan."

Ryan is Australian Ryan McNaught, a Lego certified professional. And Mr McNaught has a secret for any visitors to the structure.

The tree is 10 metres tall and took 1200 hours to build. Photo: Michele Mossop

"When people look at the tree, they should have a closer look," he said. "Throughout the whole set, there are little secret touches. There might be a couple of visitors from a galaxy far far away."

Mr McNaught has built a number of models, including the Sydney Opera House, and most recently, a 70,000-brick model of the Melbourne Cricket Ground.

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However, the Christmas tree is the biggest model he has completed.

"It's my biggest build to date and I hope it inspires Lego fans of all ages to build their own Christmas memories."

The tree was set up on Sunday night and city workers crowded around the huge structure yesterday.

"I have been to Legoland [in southern California] and seen a lot of Lego structures, and this is impressive," said Bianco Einemo, 25, from Surry Hills. "It puts you in a good mood because it reminds you of being a child again."

The unveiling tonight will be preceded by a musical performance by 400 singers from the Sydney Philharmonic Choir from 7.05.